This invention relates to a radio receiver comprising a phase locked loop frequency synthesizer for producing a local oscillator frequency signal.
In a phase locked loop frequency synthesizer used in FM radio receivers, as is well known, an output frequency f.sub.o of a voltage-controlled oscillator (VCO) acting as a local oscillator is divided down by a factor of N (an integer) in a programmable counter, an output signal of the programmable counter is compared in phase with a reference frequency signal in a phase comparator, and a control voltage having a magnitude corresponding to a phase difference between the programmable counter output signal and the reference frequency signal is applied to the VCO so that the output frequency of the VCO is stabilized at N times the reference frequency f.sub.ref. The programmable counter has its dividing factor N changed in increments of one so that the output frequency f.sub.o of the VCO may be changed in, for example, 100 KHz steps, for the reference frequency f.sub.ref of 100 KHz or the channel spacing of 100 KHz to permit the radio receiver to search broadcast stations within a frequency band with a channel spacing of 100 KHz.
The radio receivers having such a frequency synthesizer as described above suffer from disadvantages that the adjustment of IF (intermediate frequency) offset and the reception of such a broadcast station as located in Europe and having a carrier frequency which is not an integral multiple of a channel spacing frequency such as of 100 KHz cannot be accurately performed because the local oscillator frequency f.sub.o changes discretely. Namely, because the center frequency of ceramic filters or the like used in radio receivers as the intermediate frequency filter differs from filter to filter it is necessary to adjust the intermediate frequency of each radio receiver to the center frequency of the filter used therein. However, the above described discrete change in the local oscillator frequency makes the proper adjustment of the intermediate frequency of the receivers impossible. As a result, the value N in the programmable counter obtained when the receiver completely tunes to a station would not accurately correspond to the station frequency.